'\" et
.TH REGCOMP "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
regcomp,
regerror,
regexec,
regfree
\(em regular expression matching
.SH SYNOPSIS
.LP
.nf
#include <regex.h>
.P
int regcomp(regex_t *restrict \fIpreg\fP, const char *restrict \fIpattern\fP,
    int \fIcflags\fP);
size_t regerror(int \fIerrcode\fP, const regex_t *restrict \fIpreg\fP,
    char *restrict \fIerrbuf\fP, size_t \fIerrbuf_size\fP);
int regexec(const regex_t *restrict \fIpreg\fP, const char *restrict \fIstring\fP,
    size_t \fInmatch\fP, regmatch_t \fIpmatch\fP[restrict], int \fIeflags\fP);
void regfree(regex_t *\fIpreg\fP);
.fi
.SH DESCRIPTION
These functions interpret
.IR basic
and
.IR extended
regular expressions as described in the Base Definitions volume of POSIX.1\(hy2017,
.IR "Chapter 9" ", " "Regular Expressions".
.P
The
.BR regex_t
structure is defined in
.IR <regex.h> 
and contains at least the following member:
.TS
center box tab(!);
cB | cB | cB
lw(1.25i)B | lw(1.25i)I | lw(2.5i).
Member Type!Member Name!Description
_
size_t!re_nsub!T{
Number of parenthesized subexpressions.
T}
.TE
.P
The
.BR regmatch_t
structure is defined in
.IR <regex.h> 
and contains at least the following members:
.TS
center box tab(!);
cB | cB | cB
lw(1.25i)B | lw(1.25i)I | lw(2.5i).
Member Type!Member Name!Description
_
regoff_t!rm_so!T{
Byte offset from start of \fIstring\fP to start of substring.
T}
regoff_t!rm_eo!T{
Byte offset from start of
.IR string
of the first character after the end of substring.
T}
.TE
.P
The
\fIregcomp\fR()
function shall compile the regular expression contained in the string
pointed to by the
.IR pattern
argument and place the results in the structure pointed to by
.IR preg .
The
.IR cflags
argument is the bitwise-inclusive OR of zero or more of the following
flags, which are defined in the
.IR <regex.h> 
header:
.IP REG_EXTENDED 14
Use Extended Regular Expressions.
.IP REG_ICASE 14
Ignore case in match (see the Base Definitions volume of POSIX.1\(hy2017,
.IR "Chapter 9" ", " "Regular Expressions").
.IP REG_NOSUB 14
Report only success/fail in
\fIregexec\fR().
.IP REG_NEWLINE 14
Change the handling of
<newline>
characters, as described in the text.
.P
The default regular expression type for
.IR pattern
is a Basic Regular Expression. The application can specify Extended
Regular Expressions using the REG_EXTENDED
.IR cflags
flag.
.P
If the REG_NOSUB flag was not set in
.IR cflags ,
then
\fIregcomp\fR()
shall set
.IR re_nsub
to the number of parenthesized subexpressions (delimited by
.BR \(dq\e(\e)\(dq 
in basic regular expressions or
.BR \(dq(\|)\(dq 
in extended regular expressions) found in
.IR pattern .
.P
The
\fIregexec\fR()
function compares the null-terminated string specified by
.IR string
with the compiled regular expression
.IR preg
initialized by a previous call to
\fIregcomp\fR().
If it finds a match,
\fIregexec\fR()
shall return 0; otherwise, it shall return non-zero indicating either
no match or an error. The
.IR eflags
argument is the bitwise-inclusive OR of zero or more of the following
flags, which are defined in the
.IR <regex.h> 
header:
.IP REG_NOTBOL 14
The first character of the string pointed to by
.IR string
is not the beginning of the line. Therefore, the
<circumflex>
character
(\c
.BR '\(ha' ),
when taken as a special character, shall not match the beginning of
.IR string .
.IP REG_NOTEOL 14
The last character of the string pointed to by
.IR string
is not the end of the line. Therefore, the
<dollar-sign>
(\c
.BR '$' ),
when taken as a special character, shall not match the end of
.IR string .
.P
If
.IR nmatch
is 0 or REG_NOSUB was set in the
.IR cflags
argument to
\fIregcomp\fR(),
then
\fIregexec\fR()
shall ignore the
.IR pmatch
argument. Otherwise, the application shall ensure that the
.IR pmatch
argument points to an array with at least
.IR nmatch
elements, and
\fIregexec\fR()
shall fill in the elements of that array with offsets of the substrings
of
.IR string
that correspond to the parenthesized subexpressions of
.IR pattern :
.IR pmatch [\c
.IR i ].\c
.IR rm_so
shall be the byte offset of the beginning and
.IR pmatch [\c
.IR i ].\c
.IR rm_eo
shall be one greater than the byte offset of the end of substring
.IR i .
(Subexpression
.IR i
begins at the
.IR i th
matched open parenthesis, counting from 1.) Offsets in
.IR pmatch [0]
identify the substring that corresponds to the entire regular
expression. Unused elements of
.IR pmatch
up to
.IR pmatch [\c
.IR nmatch \-1]
shall be filled with \-1. If there are more than
.IR nmatch
subexpressions in
.IR pattern
(\c
.IR pattern
itself counts as a subexpression), then
\fIregexec\fR()
shall still do the match, but shall record only the first
.IR nmatch
substrings.
.P
When matching a basic or extended regular expression, any given
parenthesized subexpression of
.IR pattern
might participate in the match of several different substrings of
.IR string ,
or it might not match any substring even though the pattern as a whole
did match. The following rules shall be used to determine which
substrings to report in
.IR pmatch
when matching regular expressions:
.IP " 1." 4
If subexpression
.IR i
in a regular expression is not contained within another subexpression,
and it participated in the match several times, then the byte offsets
in
.IR pmatch [\c
.IR i ]
shall delimit the last such match.
.IP " 2." 4
If subexpression
.IR i
is not contained within another subexpression, and it did not
participate in an otherwise successful match, the byte offsets in
.IR pmatch [\c
.IR i ]
shall be \-1. A subexpression does not participate in the match when:
.sp
.RS
.BR '*' 
or
.BR \(dq\e{\e}\(dq 
appears immediately after the subexpression in a basic regular
expression, or
.BR '*' ,
.BR '?' ,
or
.BR \(dq{\|}\(dq 
appears immediately after the subexpression in an extended regular
expression, and the subexpression did not match (matched 0 times)
.RE
.RS 4 
.P
or:
.sp
.RS
.BR '|' 
is used in an extended regular expression to select this subexpression
or another, and the other subexpression matched.
.RE
.RE
.IP " 3." 4
If subexpression
.IR i
is contained within another subexpression
.IR j ,
and
.IR i
is not contained within any other subexpression that is contained
within
.IR j ,
and a match of subexpression
.IR j
is reported in
.IR pmatch [\c
.IR j ],
then the match or non-match of subexpression
.IR i
reported in
.IR pmatch [\c
.IR i ]
shall be as described in 1. and 2. above, but within the substring
reported in
.IR pmatch [\c
.IR j ]
rather than the whole string. The offsets in
.IR pmatch [\c
.IR i ]
are still relative to the start of
.IR string .
.IP " 4." 4
If subexpression
.IR i
is contained in subexpression
.IR j ,
and the byte offsets in
.IR pmatch [\c
.IR j ]
are \-1, then the pointers in
.IR pmatch [\c
.IR i ]
shall also be \-1.
.IP " 5." 4
If subexpression
.IR i
matched a zero-length string, then both byte offsets in
.IR pmatch [\c
.IR i ]
shall be the byte offset of the character or null terminator
immediately following the zero-length string.
.P
If, when
\fIregexec\fR()
is called, the locale is different from when the regular expression was
compiled, the result is undefined.
.P
If REG_NEWLINE is not set in
.IR cflags ,
then a
<newline>
in
.IR pattern
or
.IR string
shall be treated as an ordinary character. If REG_NEWLINE is set, then
<newline>
shall be treated as an ordinary character except as follows:
.IP " 1." 4
A
<newline>
in
.IR string
shall not be matched by a
<period>
outside a bracket expression or by any form of a non-matching list
(see the Base Definitions volume of POSIX.1\(hy2017,
.IR "Chapter 9" ", " "Regular Expressions").
.IP " 2." 4
A
<circumflex>
(\c
.BR '\(ha' )
in
.IR pattern ,
when used to specify expression anchoring (see the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 9.3.8" ", " "BRE Expression Anchoring"),
shall match the zero-length string immediately after a
<newline>
in
.IR string ,
regardless of the setting of REG_NOTBOL.
.IP " 3." 4
A
<dollar-sign>
(\c
.BR '$' )
in
.IR pattern ,
when used to specify expression anchoring, shall match the zero-length
string immediately before a
<newline>
in
.IR string ,
regardless of the setting of REG_NOTEOL.
.P
The
\fIregfree\fR()
function frees any memory allocated by
\fIregcomp\fR()
associated with
.IR preg .
.P
The following constants are defined as the minimum set of error return
values, although other errors listed as implementation extensions in
.IR <regex.h> 
are possible:
.IP REG_BADBR 14
Content of
.BR \(dq\e{\e}\(dq 
invalid: not a number, number too large, more than two numbers, first
larger than second.
.IP REG_BADPAT 14
Invalid regular expression.
.IP REG_BADRPT 14
.BR '?' ,
.BR '*' ,
or
.BR '+' 
not preceded by valid regular expression.
.IP REG_EBRACE 14
.BR \(dq\e{\e}\(dq 
imbalance.
.IP REG_EBRACK 14
.BR \(dq[]\(dq 
imbalance.
.IP REG_ECOLLATE 14
Invalid collating element referenced.
.IP REG_ECTYPE 14
Invalid character class type referenced.
.IP REG_EESCAPE 14
Trailing
<backslash>
character in pattern.
.IP REG_EPAREN 14
.BR \(dq\e(\e)\(dq 
or
.BR \(dq()\(dq 
imbalance.
.IP REG_ERANGE 14
Invalid endpoint in range expression.
.IP REG_ESPACE 14
Out of memory.
.IP REG_ESUBREG 14
Number in
.BR \(dq\edigit\(dq 
invalid or in error.
.IP REG_NOMATCH 14
\fIregexec\fR()
failed to match.
.P
If more than one error occurs in processing a function call, any one
of the possible constants may be returned, as the order of detection is
unspecified.
.P
The
\fIregerror\fR()
function provides a mapping from error codes returned by
\fIregcomp\fR()
and
\fIregexec\fR()
to unspecified printable strings. It generates a string corresponding
to the value of the
.IR errcode
argument, which the application shall ensure is the last non-zero value
returned by
\fIregcomp\fR()
or
\fIregexec\fR()
with the given value of
.IR preg .
If
.IR errcode
is not such a value, the content of the generated string is unspecified.
.P
If
.IR preg
is a null pointer, but
.IR errcode
is a value returned by a previous call to
\fIregexec\fR()
or
\fIregcomp\fR(),
the
\fIregerror\fR()
still generates an error string corresponding to the value of
.IR errcode ,
but it might not be as detailed under some implementations.
.P
If the
.IR errbuf_size
argument is not 0,
\fIregerror\fR()
shall place the generated string into the buffer of size
.IR errbuf_size
bytes pointed to by
.IR errbuf .
If the string (including the terminating null) cannot fit in the
buffer,
\fIregerror\fR()
shall truncate the string and null-terminate the result.
.P
If
.IR errbuf_size
is 0,
\fIregerror\fR()
shall ignore the
.IR errbuf
argument, and return the size of the buffer needed to hold the
generated string.
.P
If the
.IR preg
argument to
\fIregexec\fR()
or
\fIregfree\fR()
is not a compiled regular expression returned by
\fIregcomp\fR(),
the result is undefined. A
.IR preg
is no longer treated as a compiled regular expression after it is given
to
\fIregfree\fR().
.SH "RETURN VALUE"
Upon successful completion, the
\fIregcomp\fR()
function shall return 0. Otherwise, it shall return an integer value
indicating an error as described in
.IR <regex.h> ,
and the content of
.IR preg
is undefined. If a code is returned, the interpretation shall be as
given in
.IR <regex.h> .
.P
If
\fIregcomp\fR()
detects an invalid RE, it may return REG_BADPAT, or it may return one
of the error codes that more precisely describes the error.
.P
Upon successful completion, the
\fIregexec\fR()
function shall return 0. Otherwise, it shall return REG_NOMATCH to
indicate no match.
.P
Upon successful completion, the
\fIregerror\fR()
function shall return the number of bytes needed to hold the entire
generated string, including the null termination. If the return value
is greater than
.IR errbuf_size ,
the string returned in the buffer pointed to by
.IR errbuf
has been truncated.
.P
The
\fIregfree\fR()
function shall not return a value.
.SH ERRORS
No errors are defined.
.LP
.IR "The following sections are informative."
.SH "EXAMPLES"
.sp
.RS 4
.nf

#include <regex.h>
.P
/*
 * Match string against the extended regular expression in
 * pattern, treating errors as no match.
 *
 * Return 1 for match, 0 for no match.
 */
.P
int
match(const char *string, char *pattern)
{
    int    status;
    regex_t    re;
.P
    if (regcomp(&re, pattern, REG_EXTENDED|REG_NOSUB) != 0) {
        return(0);      /* Report error. */
    }
    status = regexec(&re, string, (size_t) 0, NULL, 0);
    regfree(&re);
    if (status != 0) {
        return(0);      /* Report error. */
    }
    return(1);
}
.fi
.P
.RE
.P
The following demonstrates how the REG_NOTBOL flag could be used with
\fIregexec\fR()
to find all substrings in a line that match a pattern supplied by a user.
(For simplicity of the example, very little error checking is done.)
.sp
.RS 4
.nf

(void) regcomp (&re, pattern, 0);
/* This call to regexec() finds the first match on the line. */
error = regexec (&re, &buffer[0], 1, &pm, 0);
while (error == 0) {  /* While matches found. */
    /* Substring found between pm.rm_so and pm.rm_eo. */
    /* This call to regexec() finds the next match. */
    error = regexec (&re, buffer + pm.rm_eo, 1, &pm, REG_NOTBOL);
}
.fi
.P
.RE
.SH "APPLICATION USAGE"
An application could use:
.sp
.RS 4
.nf

regerror(code,preg,(char *)NULL,(size_t)0)
.fi
.P
.RE
.P
to find out how big a buffer is needed for the generated string,
\fImalloc\fR()
a buffer to hold the string, and then call
\fIregerror\fR()
again to get the string. Alternatively, it could allocate a fixed,
static buffer that is big enough to hold most strings, and then use
\fImalloc\fR()
to allocate a larger buffer if it finds that this is too small.
.P
To match a pattern as described in the Shell and Utilities volume of POSIX.1\(hy2017,
.IR "Section 2.13" ", " "Pattern Matching Notation",
use the
\fIfnmatch\fR()
function.
.SH RATIONALE
The
\fIregexec\fR()
function must fill in all
.IR nmatch
elements of
.IR pmatch ,
where
.IR nmatch
and
.IR pmatch
are supplied by the application, even if some elements of
.IR pmatch
do not correspond to subexpressions in
.IR pattern .
The application developer should note that there is probably no reason
for using a value of
.IR nmatch
that is larger than
.IR preg \->\c
.IR re_nsub +1.
.P
The REG_NEWLINE flag supports a use of RE matching that is needed in
some applications like text editors. In such applications, the user
supplies an RE asking the application to find a line that matches the
given expression. An anchor in such an RE anchors at the beginning or
end of any line. Such an application can pass a sequence of
<newline>-separated
lines to
\fIregexec\fR()
as a single long string and specify REG_NEWLINE to
\fIregcomp\fR()
to get the desired behavior. The application must ensure that there are
no explicit
<newline>
characters in
.IR pattern
if it wants to ensure that any match occurs entirely within a single
line.
.P
The REG_NEWLINE flag affects the behavior of
\fIregexec\fR(),
but it is in the
.IR cflags
parameter to
\fIregcomp\fR()
to allow flexibility of implementation. Some implementations will want
to generate the same compiled RE in
\fIregcomp\fR()
regardless of the setting of REG_NEWLINE and have
\fIregexec\fR()
handle anchors differently based on the setting of the flag. Other
implementations will generate different compiled REs based on the
REG_NEWLINE.
.P
The REG_ICASE flag supports the operations taken by the
.IR grep
.BR \-i
option and the historical implementations of
.IR ex
and
.IR vi .
Including this flag will make it easier for application code to be
written that does the same thing as these utilities.
.P
The substrings reported in
.IR pmatch [\|]
are defined using offsets from the start of the string rather than
pointers. This allows type-safe access to both constant and non-constant
strings.
.P
The type
.BR regoff_t
is used for the elements of
.IR pmatch [\|]
to ensure that the application can represent large arrays in memory
(important for an application conforming to the Shell and Utilities volume of POSIX.1\(hy2017).
.P
The 1992 edition of this standard required
.BR regoff_t
to be at least as wide as
.BR off_t ,
to facilitate future extensions in which the string to be searched is
taken from a file. However, these future extensions have not appeared.
The requirement rules out popular implementations with 32-bit
.BR regoff_t
and 64-bit
.BR off_t ,
so it has been removed.
.P
The standard developers rejected the inclusion of a
\fIregsub\fR()
function that would be used to do substitutions for a matched RE. While
such a routine would be useful to some applications, its utility would
be much more limited than the matching function described here. Both RE
parsing and substitution are possible to implement without support
other than that required by the ISO\ C standard, but matching is much more
complex than substituting. The only difficult part of substitution,
given the information supplied by
\fIregexec\fR(),
is finding the next character in a string when there can be multi-byte
characters. That is a much larger issue, and one that needs a more
general solution.
.P
The
.IR errno
variable has not been used for error returns to avoid filling the
.IR errno
name space for this feature.
.P
The interface is defined so that the matched substrings
.IR rm_sp
and
.IR rm_ep
are in a separate
.BR regmatch_t
structure instead of in
.BR regex_t .
This allows a single compiled RE to be used simultaneously in several
contexts; in
\fImain\fR()
and a signal handler, perhaps, or in multiple threads of lightweight
processes. (The
.IR preg
argument to
\fIregexec\fR()
is declared with type
.BR const ,
so the implementation is not permitted to use the structure to store
intermediate results.) It also allows an application to request an
arbitrary number of substrings from an RE. The number of
subexpressions in the RE is reported in
.IR re_nsub
in
.IR preg .
With this change to
\fIregexec\fR(),
consideration was given to dropping the REG_NOSUB flag since the user
can now specify this with a zero
.IR nmatch
argument to
\fIregexec\fR().
However, keeping REG_NOSUB allows an implementation to use a different
(perhaps more efficient) algorithm if it knows in
\fIregcomp\fR()
that no subexpressions need be reported. The implementation is only
required to fill in
.IR pmatch
if
.IR nmatch
is not zero and if REG_NOSUB is not specified. Note that the
.BR size_t
type, as defined in the ISO\ C standard, is unsigned, so the description of
\fIregexec\fR()
does not need to address negative values of
.IR nmatch .
.P
REG_NOTBOL was added to allow an application to do repeated searches
for the same pattern in a line. If the pattern contains a
<circumflex>
character that should match the beginning of a line, then the pattern
should only match when matched against the beginning of the line.
Without the REG_NOTBOL flag, the application could rewrite the
expression for subsequent matches, but in the general case this would
require parsing the expression. The need for REG_NOTEOL is not as
clear; it was added for symmetry.
.P
The addition of the
\fIregerror\fR()
function addresses the historical need for conforming application
programs to have access to error information more than ``Function
failed to compile/match your RE for unknown reasons''.
.P
This interface provides for two different methods of dealing with error
conditions. The specific error codes (REG_EBRACE, for example), defined
in
.IR <regex.h> ,
allow an application to recover from an error if it is so able. Many
applications, especially those that use patterns supplied by a user,
will not try to deal with specific error cases, but will just use
\fIregerror\fR()
to obtain a human-readable error message to present to the user.
.P
The
\fIregerror\fR()
function uses a scheme similar to
\fIconfstr\fR()
to deal with the problem of allocating memory to hold the generated
string. The scheme used by
\fIstrerror\fR()
in the ISO\ C standard was considered unacceptable since it creates difficulties
for multi-threaded applications.
.P
The
.IR preg
argument is provided to
\fIregerror\fR()
to allow an implementation to generate a more descriptive message than
would be possible with
.IR errcode
alone. An implementation might, for example, save the character offset
of the offending character of the pattern in a field of
.IR preg ,
and then include that in the generated message string. The
implementation may also ignore
.IR preg .
.P
A REG_FILENAME flag was considered, but omitted. This flag caused
\fIregexec\fR()
to match patterns as described in the Shell and Utilities volume of POSIX.1\(hy2017,
.IR "Section 2.13" ", " "Pattern Matching Notation"
instead of REs. This service is now provided by the
\fIfnmatch\fR()
function.
.P
Notice that there is a difference in philosophy between the ISO\ POSIX\(hy2:\|1993 standard and
POSIX.1\(hy2008 in how to handle a ``bad'' regular expression. The ISO\ POSIX\(hy2:\|1993 standard says
that many bad constructs ``produce undefined results'', or that
``the interpretation is undefined''. POSIX.1\(hy2008, however, says that the
interpretation of such REs is unspecified. The term ``undefined'' means
that the action by the application is an error, of similar severity
to passing a bad pointer to a function.
.P
The
\fIregcomp\fR()
and
\fIregexec\fR()
functions are required to accept any null-terminated string as the
.IR pattern
argument. If the meaning of the string is ``undefined'', the behavior
of the function is ``unspecified''. POSIX.1\(hy2008 does not specify how the
functions will interpret the pattern; they might return error codes, or
they might do pattern matching in some completely unexpected way, but
they should not do something like abort the process.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIfnmatch\fR\^(\|)",
.IR "\fIglob\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Chapter 9" ", " "Regular Expressions",
.IR "\fB<regex.h>\fP",
.IR "\fB<sys_types.h>\fP"
.P
The Shell and Utilities volume of POSIX.1\(hy2017,
.IR "Section 2.13" ", " "Pattern Matching Notation"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
